1. Field of Invention
The invention relates to a developing apparatus provided to an image forming apparatus, such as a facsimile machine or a laser printer, and relates to an image forming apparatus provided with a developing apparatus.
2. Description of Related Art
A color laser printer using non-magnetic single-component toner has a plurality of developing cartridges, each having a developing roller that carries toner, and a layer thickness-regulating blade that is disposed at a side of the developing roller and presses a surface of the developing roller to form a thin layer of toner. In each of the developing cartridges, the layer thickness-regulating blade is pressed against the surface of the associated developing roller. Color toner supplied from the developing roller is sufficiently charged by being rubbed between the layer thickness-regulating blade and the developing roller, and is carried, as a thin layer of toner, on the developing roller.
In addition, in such a laser printer, a photosensitive drum is disposed so as to face the developing roller of each of the developing cartridges. A surface of the photosensitive drum is uniformly charged by a charger. After that, a laser beam emitted from a laser emitter, based on a predetermined image data, is scanned at high speed over the surface of the photosensitive drum, and an electrostatic latent image is formed thereon.
Then, the developing roller of each of the developing cartridges sequentially comes to face the photosensitive drum, and an electrostatic latent image formed on the photosensitive drum is developed, using the toner of each color, into a visible image. Then, the visible image of each color is sequentially transferred to an intermediate transfer drum to form a multicolored image. Finally, the multicolored image is transferred at a time from the intermediate transfer drum to a sheet of paper.
For reproduction of a high-quality halftone image, it is necessary to reduce variations in density of toner, carried on the surface of the developing roller, that is caused by different development histories of toner. Such variations in density are produced because the density of a halftone visible image to be developed when a thickness-regulated layer of toner remains on the surface of the developing roller without having been used for developing the entire surface of the photosensitive drum (when a blank image has been developed on the entire surface of the photosensitive drum) defers the density of a halftone visible image to be developed when only new toner is supplied and carried, as a thickness-regulated layer, on the surface of the developing roller after the entire surface of the photosensitive drum has been developed using toner (when a color image has been developed on the entire surface of the photosensitive drum).
To eliminate such variations in density, it is necessary to equate the charge amount per unit weight q1 of the toner, which is carried, as a thickness-regulated layer, on the surface of the developing roller after a blank image has been developed on the entire surface of the photosensitive drum, with the charge amount per unit weight q2 of the toner, which is carried, as a thickness-regulated layer, on the surface of the developing roller after a color image has been developed on the entire surface of the photosensitive drum.
After a blank image has been developed on the entire surface of the photosensitive drum, already charged toner remains on the developing roller without having been consumed. Thus, at the next development, a visible image is developed using the remaining toner that is further stably charged. On the other hand, no toner remains on the developing roller after a color image has been developed on the entire surface of the photosensitive drum and, at the next development, a visible image is developed using the toner that is newly supplied to the developing roller and newly charged. Thus, as the former toner and the latter toner have different development histories, it is hard to equate their charge amounts.
Particularly, when color toner other than carbon black is used, it is hard to reduce variations in density because the color toner is slow to be charged, and the charge level will not be stabilized.
Further, the above-described variations in density are produced not only when the entire surface of the photosensitive drum is developed but also when partially solidly shaded portions are developed, as in the case where characters having a large font size are printed.
Therefore, the invention aims at producing a developing apparatus that can minimize variations in density caused by different development histories and can reproduce a high-quality halftone image.
In a developing device according to the invention, a non-magnetic single-component developing agent is used, and the charge amount of the developing agent carried, as a thin layer, on a developing agent carrier is adjusted to become substantially constant. With this adjustment, variations in density caused by different development histories can be reduced.
Particularly, when m1 represents a weight per unit area of a thin layer of the developing agent that is pressed again by a layer thickness-regulating member while having been carried on the surface of the developing agent carrier that corresponds to an area of a photosensitive member where no visible image has been formed, and m2 represents the weight per unit area of a thin layer of developing agent that is pressed by the layer thickness-regulating member after being newly supplied to the surface of the developing agent carrier that corresponds to an area of the photosensitive member where a solidly shaded visible image has been formed, the development apparatus is constructed so as to satisfy an equation that m2=cm1 (0.75xe2x89xa6cxe2x89xa60.95). Further, it is more preferable that c is within a range of 0.80 to 0.90 (0.80xe2x89xa6cxe2x89xa60.90).
When polymerized toner is used as the developing agent, flowability of the developing agent is improved. Thus, the developing agent is stably supplied to the developing agent carrier and solidly shaded images can be formed continuously.
When the above-described developing apparatus of the invention is used as a developing apparatus of a color laser printer, where color toner not containing carbon black as a charge control agent, for example, cyan, magenta, and yellow toner are used, effects produced uniquely by the invention become remarkable. This is because the toner not containing carbon black is slow to be charged and takes time to be charged to a certain level. Thus, particularly, a thin layer of the developing agent, which is pressed by the layer thickness-regulating member after being newly supplied to a surface of the developing agent carrier that corresponds to an area of the photosensitive member where a solidly shaded visible image has been formed, is not charged enough to be deposited on a subsequent electrostatic latent image on the photosensitive member. As a result, a ghost image is produced.
If the developing apparatus according to the invention is used for a color laser printer, production of such a ghost image can effectively be prevented.